In recent years, secondary batteries such as lithium ion secondary batteries have been developed as high-energy density batteries. Secondary batteries are expected as power sources for vehicles such as hybrid automobiles and electric automobiles, and as large-size power sources for electric storage. In particular, for the application for vehicles, secondary batteries are required to have other performance such as rapid charge-and-discharge performance and long-term reliability. Secondary batteries capable of rapid charge and discharge have the advantage of a very short charging time. Furthermore, such a battery can improve performances related to motive force and further efficiently recover a regenerative energy from motive force, in a hybrid vehicle or the like.
The rapid charge and discharge are made possible by rapid movements of electrons and lithium ions between positive electrodes and negative electrodes. However, when a battery using a carbon-based negative electrode is repeatedly subjected to rapid charge-and-discharge, dendrite precipitation of metal lithium may occur on the electrode. These dendrites can cause internal short-circuits, thereby resulting in heat generation and ignition.
Therefore, batteries have been developed which use, as negative electrode active materials, metal composite oxides in place of the carbonaceous materials. In particular, batteries that use titanium-containing oxides as negative electrode active materials are capable of stable rapid charge and discharge, and also exhibit longer lifetimes as compared with carbon-based negative electrodes.
However, such titanium-containing oxides have higher (nobler) potentials with respect to metal lithium as compared with carbonaceous materials. Moreover, the titanium-containing oxides are low in capacity per mass. For these reasons, batteries using these titanium-containing oxides have the problem of being low in energy density.
For example, the operating potential of an electrode including a spinel-type lithium titanate Li4Ti5O12 is approximately 1.5 V on the basis of metal lithium, and higher (nobler) as compared with the potentials of carbon-based negative electrodes. The potential of the lithium titanate is derived from a redox reaction between Ti3+ and Ti4+ in the electrochemical insertion and extraction of lithium, and thus electrochemically restricted. In addition, rapid lithium ion charging and discharging can be achieved in a stable manner at a high electrode potential on the order of 1.5 V (vs. Li/Li+). Therefore, it is substantially difficult to lower the potential of the electrode including the lithium titanate in order to improve the energy density.
On the other hand, as for the capacity per unit mass, lithium-titanium composite oxides such as the spinel-type lithium titanate Li4Ti5O12 have a theoretical capacity of around 175 mAh/g. On the other hand, common carbon-based electrode materials have a theoretical capacity of 372 mAh/g. Accordingly, titanium-containing oxides are significantly lower in capacity density as compared with carbon-based negative electrodes. This is because the crystal structures of the titanium-containing oxides have small numbers of sites in which lithium can be inserted, or lithium is easily stabilized in the structures, thus decreasing the substantial capacity.